The present application claims priority to Japanese Application No. P11-192709 filed Jul. 7, 1999 which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to an electroless plating method and an electroless plating solution. More specifically, it relates to an electroless plating method and an electroless plating solution which are suited for copper wiring by copper plating of contact holes or damascene interconnection in, for example, a semiconductor integrated circuit device.
2. Description of the Related Art
An aluminum alloy has been so far used as a fine wiring material of a high-density integrated circuit formed on a semiconductor wafer (hereinafter referred to as a semiconductor device). However, for further increasing the speed of the semiconductor device, copper or silver having a lower specific resistance has to be used as a wiring material.
Especially, copper has a specific resistance which is as low as 1.8 xcexcxcfx89-cm, and it is thus advantageous for increasing the speed of the semiconductor device. Moreover, copper has a resistance to electromigration which is higher than that of an aluminum alloy by approximately one figure. Accordingly, it has been expected to be a material in the next generation.
A large number of contact holes or via holes (hereinafter referred to as contact holes) through which to electrically connect elements or multilayer wirings are formed in a semiconductor device. Contact holes are ordinarily formed by forming openings in interlayer insulation layers and embedding a conductive material therein.
In recent years, a single damascene method in which a groove is formed in the interlayer insulation layer and copper is embedded in the groove to form a groove wiring or a dual damascene method in which copper is embedded in a groove and an opening provided in the bottom of the groove to integrally form the groove wiring and the contact hole is being put to practical use.
As a method in which copper is embedded in a contact hole with good precision, an electroplating method has attracted much interest in recent years. A copper film formed by the electroplating method has a low impurity concentration therein and also a low resistance. Accordingly, the copper film is advantageous for increasing the speed of the semiconductor device. However, the embedding property of copper in the contact hole is greatly dependent on a step coverage of an undercoat layer (seed layer) required when forming a copper layer by an electroplating method. That is, when embedding copper in the contact hole by the electroplating method, it is required that the step coverage of the seed layer is good enough.
A copper layer formed by a sputtering method and having a thickness of approximately 100 nm has been so far used as a seed layer. Nevertheless, the step coverage of the seed layer formed by the sputtering method is, in many cases, not so good, making it difficult to uniformly form the seed layer in the contact hole. Further, when an aspect ratio of the contact hole (diameter to depth ratio of the opening of the contact hole) is more than 1:5, it is almost impossible to provide a uniform step coverage.
Accordingly, an attempt has been made to improve the step coverage by forming a seed layer through electroless plating (chemical reduction plating) of copper.
The electroless plating of copper is, however, problematic in that because of the low catalytic activity of copper itself, a film thickness required as a seed layer is hardly provided and a life of an electroless plating solution is short. Further, since copper particles are coarse, there is a problem that small voids (hollow portions) are formed between copper particles inside the film formed.
Under these circumstances, the invention aims to provide an electroless plating method and an electroless plating solution in which a good-quality film can be formed using copper in a portion having a high aspect ratio, such as a contact hole of a semiconductor device.
That is, the invention relates to an electroless plating method (hereinafter referred to as a plating method of the invention) in which when a surface of an activated product to be plated is subjected to electroless plating of copper using an electroless plating solution containing a copper salt, a chelating agent and a reducing agent, a salt of a metal such as gold, nickel, palladium, cobalt or platinum is added to the electroless plating solution as a plating accelerator in an amount of 1 mol % or less based on the copper salt in the composition of the electroless plating solution.
According to the plating method of the invention, the salt of the metal having a high catalytic activity, such as gold, nickel, palladium, cobalt or platinum is added as the plating accelerator in the specific amount of 1 mol % or less based on the copper salt in the composition of the electroless plating solution. Consequently, it is possible that at the initial stage of the electroless plating, the appropriate amount of the metal (plating accelerator) having the higher catalytic activity than copper is precipitated on the surface of the product to be plated before copper is precipitated and copper is then precipitated thereon. Thus, copper can uniformly be precipitated to form a good-quality copper-plated film. On the contrary, unless the plating accelerator is added, the copper-plated film is not uniformly precipitated because voids are formed. Further, when the amount of the accelerator exceeds 1 mol %, the copper-plated film is abnormally precipitated. At any rate, no uniform copper plating can be applied.
Moreover, the invention further provides, as a plating solution suited for practicing the plating method, an electroless plating solution (hereinafter referred to as a plating solution of the invention) in which in addition to a copper salt, a chelating agent and a reducing agent, a salt of a metal such as gold, nickel, palladium, cobalt or platinum is added as an accelerator of the electroless plating in an amount of 1 mol % or less based on the copper salt in the composition of the plating solution for subjecting a surface of an activated product to be plated to the electroless plating of copper.